This invention relates to the drying of fruit pulp and the like and, more particularly, to the efficient transfer of heat to the material being dried.
Most presently used processes for drying fruit pulp to produce, for example, fruit leathers or concentrates of the pulp, involve placing the pulp on a carrier and then heating the carrier to dry the pulp. In these systems the entire carrier is heated, regardless of whether it contains pulp or not and a substantial heat transfer occurs from the carrier to the air in places on the carrier where there is no pulp, without any appreciable use of that heat in the drying of the pulp. For example, in the system of Shepard, shown in U.S. Pat. No. 2,301,589, issued Nov. 10, 1942, a metallic carrier is provided and the pulp is laid on the metallic carrier. The carrier is then heated through electrical induction and the heat used to drive the moisture from the pulp. Since the entire carrier is heated, it is necessary to ensure that every portion of the carrier is covered with pulp in order to obtain the most efficient heat transfer from the carrier to the pulp. In actual practice, it is difficult if not impossible to obtain such complete coverage of the carrier and therefore heat is lost to the open air without doing any substantial work in drying the pulp, causing more heat to be consumed than is necessary simply to dry the pulp. In a system like that of Shepard's, the only way to obtain 100 percent heat transfer from the carrier medium to the pulp is to have an inspection and control station to monitor the coverage of the carrier by the pulp and adjust the pulp so that it covers every open area of the carrier. This causes additional problems in labor and monitoring controls that may be enough to offset any gain in efficiency of heating.